1. a. Field of Invention
This invention relates to the preparation of a methacrylate, and more particularly to a process for the preparation of a methacrylate by reacting methacrylic acid and an alcohol in vapor phase in the presence of a catalyst.
2. B. Description of the Prior Art
Methacrylates have been commercially prepared by reacting acetone cyanohydrin and sulfuric acid to form methacrylamide sulfate and then adding water and an alcohol to effect hydrolysis and esterification.
In the prior art, it is known to react methacrylic acid and alcohols at elevated temperatures in liquid or vapor phase in the presence of various catalysts. However, these methods have not been satisfactory due to the high polymerizability of the starting material and the product and difficulties in the separation and the purification of products. In consideration of the fact that starting material and product are highly polymerizable, the esterification should be carried out under the mildest conditions and completed within the shortest possible period of time. Therefore it is advantageous to carry out the reaction in vapor phase for a short contact time by employing a highly active catalyst.
A number of catalysts for the esterification of methacrylic acid in vapor phase have been proposed as listed below.
A. Silica gel catalyst (Japanese Patent Publication No. 17706/1961) PA1 B. Phosphoric acid - diatomaceous earth catalyst (Japanese Patent Publication No. 5222/1967) PA1 C. Silica-alumina catalyst (Japanese Patent Publication No. 6324/1967) PA1 D. Molybdenum oxide, molybdenum sulfide and sulfided cobalt-molybdate catalysts (Japanese Patent Publication No. 4332/1969) PA1 E. Molybdenum sulfide catalyst (Japanese Patent Publication No. 9885/1969) PA1 F. Titanium oxide-antimony oxide-silica catalyst (Japanese Patent Publication No. 24564/1970) PA1 G. Sulfuric acid- or sulfonic acid-carrying catalyst (U.S. Pat. No. 3,392,191) PA1 H. TiO.sub.2, TiO.sub.2 -Sb.sub.2 O.sub.5, TiO.sub.2 -SiO.sub.2 and TiO.sub.2 -Sb.sub.2 O.sub.5 -SiO.sub.2 catalysts (U.S. Pat. No. 3,639,461)
The above-listed catalysts, however, have disadvantages in that elevated temperatures are required for the reaction due to their low activity and the products can not be readily separated and purified due to low conversion and so on. In a prior art process where low selectivity of methacrylate requires the presence of water vapor in the reaction gas, the separation and purification of the product is difficult. Moreover, some catalysts are unstable and their initial activity disappears after a short period of time and thus are not suitable for industrial use.